Phase shifters have a wide variety of uses in current and next generation communication systems. Phase shifters are used to adjust the transmission phase of RF signals in beam-forming, in-phase modulation, and power amplifier circuits, for example. Fifth generation (5G) wireless systems represent one emerging application for phase shifters. Fifth generation (5G) wireless systems use a plurality of antennas and phase shifting to simultaneously transmit and receive multiple RF signals and thereby achieve faster download speeds, greater bandwidth, spectral efficiency, lower latency, etc.
Generally speaking, phase shifters fall into two broad categories: active phase shifters and passive phase shifters. Passive phase shifters include only passive elements (e.g., inductors, transmission lines, etc.) whereas active phase shifters use at least one active element (e.g., transistors, thyristors, etc.). Active phase shifters consume additional power in comparison to passive phase shifters. Therefore, passive phase shifters are preferable in base station and mobile device applications.
Passive phase shifters are classified into four broad categories: switched line phase shifters, transmission type phase shifters, L-C lumped element phase shifters, and reflection type phase shifters. Switched line phase shifters switch between transmission lines of different length to achieve a desired delay. Switched line phase shifters can have the drawback of high insertion loss as more stages are cascaded to increase resolution. A transmission type phase shifter is realized by loading a reactive impedance on a transmission line, and tuning the impedance value to change the phase of the RF signal. The drawback of the transmission type phase shifter is that the input impedance is difficult to match when the phase is tuning, which turns results in a high return loss. L-C lumped element phase shifters use a plurality of lumped elements (e.g., capacitors and inductors). In order to tune the phase, a variable capacitor can be substituted in place of some or all of the capacitors. One disadvantage of this type of phase shifter is that the characteristic impedance of phase shifter changes and is therefore difficult to match. A reflection type phase shifter can include a 3 dB coupler with reactive terminated load. One benefit of a reflection type phase shifter is very low return loss and matched port impedance. However, the size and bandwidth of reflection type phase shifters, which is determined by the properties of the 3 dB coupler, limits their applicability.